In modern telecommunications networks, an out-of-band common channel signaling (CCS) network is employed to facilitate the setup and tear down of voice and/or data communication sessions between telephone service subscribers. In the United States, this signaling network is referred to as the signaling system 7 (SS7) network. The SS7 network utilizes the ISDN user part (ISUP) protocol to set up and tear down calls. ISUP messages used to set up calls include an initial address message (IAM), an address complete message (ACM), and an answer message (ANM). FIG. 1 illustrates an exemplary ISUP call setup message flow. Referring to FIG. 1, a telecommunications network 100 includes a calling party 110, an originating end office 112, a first signal transfer point (STP) 114, a second STP 116, a terminating end office 118, and a called party 120. A voice grade bearer path 122 and an SS7 signaling path 124 are also utilized to facilitate the establishment of a call.
In FIG. 1, calling party 110 goes off-hook and dials the telephone number associated with called party 120. Originating end office 112 serving calling party 110 formulates an ISUP IAM message, which is transmitted via STPs 114 and 116 to terminating end office 118. Terminating end office 118 serving called party 120 receives the IAM message and responds to originating end office 112 with an ACM message. The ACM message serves as both an acknowledgement of the IAM and an indicator that a ring tone is being sent to the called party. When the called party goes off-hook to answer the call, the terminating end office 118 formulates an ANM message, which is routed back to originating end office 112.
When either party goes on-hook, that party's end office sends a release (REL) message to the other end office. In FIG. 1, it is assumed that calling party 110 goes on-hook. Accordingly, end office 112 sends a release message to end office 118. When end office 118 receives the release message, end office 118 formulates a release complete (RLC) and sends the RLC message to end office 112.
In telecommunications networks, the ANM message notifies the originating end office that the called party has answered the call and that billing for the call should be commenced. However, in some situations, an ANM message may not be returned by the terminating facility serving the called party, but the call may nonetheless be completed without the ANM message. The failure to return an ANM message during setup of a call is commonly referred to as “no answer supervision” and may be unintentional or deliberate, in order to avoid being billed for the call. For example the owner of a private branch exchange (PBX) may configure the switch in a manner such that an ANM message is not returned during the call setup signaling process in an attempt to avoid being billed for calls. Interactive voice response (IVR) systems that are widely deployed in telecommunication networks are also common sources of no answer supervision problems. Revenue losses due to no answer supervision scenarios have been and continue to be substantial for network operators.
U.S. Pat. No. 4,811,378 to Else et al. describes a technique where switches in the telephone network can be upgraded such that an open voice channel is provided only after an ANM message has been received. U.S. Pat. No. 5,265,157 to Jolissaint et al. proposes a solution to the no answer supervision problem that utilizes tonal signaling (e.g., DTMF tones) technology to ensure answer supervision. U.S. Pat. No. 6,111,946 to O'Brien discloses a method and system for providing answer supervision that involves the deployment of a special node in a telecommunications network that detects a lack of answer supervision during an in-progress call. In response to detecting a lack of answer supervision related to the call, the special node returns an ANM message to the originating end office facility on behalf of the terminating end office, PBX facility, or IVR facility. The originating end office facility may use the injected ANM message to trigger the creation of a billing record.
While the above-described solutions partially address the no answer supervision problem, each of these solutions has its disadvantages. For example, the solutions requiring end office upgrades are time and cost-intensive to implement due to the number of end offices in a service provider's network. Solutions involving in-band or DTMF signaling require specialized equipment for monitoring voice trunks and for generating in-band signaling. Finally, the solutions that involve injection of an answer message in the network require specialized trunk monitoring equipment and also create unnecessary signaling traffic in the network. Accordingly, there exists a need for improved methods and systems for identifying calls connected without answer supervision and for generating billing information for the calls.